1. Field
The present invention relates to a multilayered circuit board design support method, program, and apparatus for a multilayered circuit board provided with a through hole for component insertion, the through hole penetrating and mutually connecting solid-layer conductors disposed in a multilayer manner, and such a multilayered circuit board. Particularly, the present invention relates to a multilayered circuit board design support method, program, and apparatus for designing a connection relation of ground and power-supply solid-layer conductors with respect to the through hole so that solder risability is improved, and a multilayered circuit board designed in such a manner.
2. Description of the Related Art
Conventionally, as a printed circuit board incorporated in electronic equipment, a multilayered circuit board is used in order to increase the packing density of components and circuit patterns for downsizing. The multilayered circuit board has formed therein a through hole for solder connection with ground and power-supply solid-layer conductors disposed in inner layers and leas of surface-mounted components.
FIGS. 1A and 1B depict a conventional multilayered circuit board with its section being shown in FIG. 1A and its C-C section being shown in FIG. 1B. A multilayered circuit board 200 has a multilayer structure including, in addition to two layers on the front and back, a signal conductor 201, a power-supply solid-layer conductor 203 and ground solid-layer conductors 202, 204, 206, and 208, in which through holes 210 and 212 are provided in order to connect leads 216 and 218 of a component 214 to the ground solid-layer conductors 202, 204, 206, and 208 by solders 220 and 222.
FIG. 1B is a section of the ground solid-layer conductor 202, in which the through holes 210 and 212 penetrate the ground solid-layer conductor 202 as being integrally connected. In a solder connecting operation of the multilayered circuit board 200, with the leads 216 and 218 of the component 214 being inserted in the through holes 210 and 212, respectively, the bottom surface of the multilayered circuit board 200 is dipped in a solder melting bath so that solder rises up to the top of the through holes 210 and 212 for connection. In this case, to cause solder to sufficiently rise up to the top of the through holes 210 and 212, it is required to prevent heat from escaping from the inner-layer conductors, such as the ground solid-layer conductors 202, 204, 206, and 208. To improve solder risability that would be prevented by thermal diffusion due to connections of inner-layer conductors to the through holes, for example, a gap in a divided-ring shape, such as an inverse-flower-shaped land, is provided between a through hole and a connection portion of a conductor, thereby suppressing thermal diffusion.
However, in such a conventional multilayered circuit board, depending on higher density of components, adaptation to the environment, and other factors, it is difficult to ensure solder risability only with the connection shape that suppresses thermal diffusion from the through hole to the inner-layer conductors. That is, with the number of layers of a printed circuit board being increased, the board thickness is increased and therefore the through hole is increased. At the same time, with an increase in the number of connection of conductive layers of the power and ground solid-layer conductors to a through hole for a component terminal, thermal radiation from the through hole is increased, thereby decreasing solder risability. Also, for environmental protection, it has been made mandatory to use Pb-free solder. Pb-free solder is, for example, lead-free solder of a tin-silver-copper group. With the adoption of such material, solder risability has been decreased. Moreover, with the use of Pb-free solder, a surface treatment of forming a coating on the surface of a printed wiring board has been adopted. This surface treatment further decreases solder risability.